The present invention relates generally to semiconductor memories, and more specifically to a method and circuit for testing memory cells in a synchronous static random access memory by modulating the rate of change of voltages developed on digit lines when writing data to the memory cells.
During the manufacture of static random access memories (xe2x80x9cSRAMsxe2x80x9d), as well as other types of semiconductor memories, it is necessary to test the SRAM to ensure it is operating properly. An SRAM normally includes an array of memory cells arranged in rows and columns. The memory cells are tested by writing data to and reading data from the individual memory cells. Numerous test methodologies are utilized in testing for various types of defects that can occur in SRAM memory cells, such as shorted digit lines or inoperable access transistors. For example, a binary xe2x80x9c1xe2x80x9d or binary xe2x80x9c0xe2x80x9d may be written to and read from each SRAM memory cell, or alternating patterns of binary 1""s and 0""s may be written to and read from the memory cells in each row.
FIG. 1 illustrates a conventional SRAM memory cell 10 coupled between a pair of complementary digit lines DL and {overscore (DL)}. The SRAM memory cell 10 includes a conventional pair of cross-coupled inverters 12 and 14 comprising the transistors 16, 18 and 20, 22, respectively, coupled between a supply voltage source VCC and ground. As known in the art, each of the inverters 12 and 14 has an associated threshold voltage VT corresponding to the voltage a signal on its input must reach before the inverter begins driving its output to the complement of the input signal. A pair of access transistors 24 and 26 couple an output node 17 of the inverter 12 to the digit line {overscore (DL)} and an output node 21 of the inverter 14 to the digit line DL, respectively, in response to a word line signal WL. A write driver circuit 28 receives complementary data signals D and {overscore (D)} and develops voltages on the digit lines DL and {overscore (DL)} corresponding to the complementary data signals D and {overscore (D)}, respectively.
The operation of the SRAM memory cell 10 during a typical write operation will now be described with reference to the signal timing diagram of FIG. 2. At the time t0, the data signals D and {overscore (D)} go high and low, respectively, corresponding to the data to be stored in the memory cell 10. In response to the data signals D and {overscore (D)}, the write driver circuit 28 drives the voltages on the digit lines DL and {overscore (DL)} high and low, respectively. The write driver circuit 28 cannot instantaneously drive the voltages on the digit lines DL and {overscore (DL)} to their desired levels, however, because the digit lines DL and {overscore (DL)} are highly capacitive. The capacitances of the digit lines DL and {overscore (DL)} cause the voltages on the digit lines to gradually approach their desired levels. The word line WL is activated just after the time t0 thereby coupling the nodes 17 and 21 to the digit lines {overscore (DL)} and DL, respectively. At a time t1, the voltages on the digit lines DL and {overscore (DL)} reach the threshold voltages VT of the inverters 12 and 14, and the inverters begin driving the voltages on nodes 17 and 21 low and high, respectively. In other words, the inverter 12 begins driving the voltage on node 17 low in response to the high on the digit line DL. The voltage on node 17 is also driven low by the low going voltage on the digit line {overscore (DL)}. The inverter 14 operates in the same way to drive the voltage on node 21 high. In this way, the SRAM memory cell 10 drives the nodes 17 and 21 low and high, respectively, corresponding to the voltages developed on the digit lines {overscore (DL)} and DL. At a time t2, the word line WL is deactivated turning OFF the access transistors 24 and 26 and isolating the nodes 17 and 21 from the digit lines {overscore (DL)} and DL, respectively. The memory cell 10 maintains the voltages on the nodes 17 and 21 low and high, respectively, after the word line WL is deactivated and in this way stores the data written to the memory cell.
The threshold voltages VT of the inverters 12 and 14 determine when the inverters 12 and 14 begin turning ON and thereby determine when the memory cell 10 begins latching the data placed on the digit lines DL and {overscore (DL)}. The voltages on the digit lines DL and {overscore (DL)} must exceed the threshold voltages VT of the inverters 12 and 14 before the time t2, which is when the data signals D and {overscore (D)} change state and when the access transistors 24 and 26 are deactivated. The interval between the times t0 and t2 is the write pulse width tWPW of the data signals on the digit lines DL and {overscore (DL)}.
During the testing of SRAMs, it is desirable to perform margin tests on the SRAM memory cells in which the margins of the data signals developed on digit lines DL and {overscore (DL)} are varied. In a timing margin test, the write pulse width tWPW is made shorter than its normal duration. When the write pulse width twpw is shortened, the data signals on the digit lines DL and {overscore (DL)} may not exceed the threshold voltages VT of the inverters 12 and 14 during the write pulse width and the memory cell 10 may not latch the data placed on the digit lines DL and {overscore (DL)}. During normal operation of an SRAM slight variations in write pulse width tWPW may occur, and this type of margin test detects cells that may fail due to such slight variations. In asynchronous SRAMs, the duration of the write pulse width tWPW is controlled by external asynchronous control signals applied to the SRAM. Thus, the write pulse width tWPW can be easily adjusted by varying such external control signals. The write pulse width tWPW is shortened when writing data to the memory cells and then the data read from the memory cells to determine whether the memory cells properly stored the written data. In synchronous SRAMs, however, as well as other types of SRAMs, the write pulse width tWPW is constant and determined by a fixed internal signal that is not externally adjustable. Thus, in synchronous SRAMs, the write pulse width cannot be easily shortened to perform margin tests on the memory cells.
There is a need for performing margin tests of memory cells in synchronous SRAMs.
A write driver circuit includes a drive circuit having a first drive node adapted to receive a first voltage, a second drive node, an input adapted to receive a data signal, and an output. The drive circuit couples the output to the first drive node when the data signal has a first logic voltage, and couples the output to the second drive node when the data signal has a second logic voltage. A test circuit has an input adapted to receive a test mode signal, and an output coupled to the second drive node. The test circuit develops a first impedance between the second drive node and a second voltage source when the test mode signal is active, and develops a second impedance between the second drive node and the second voltage source when the test mode signal is inactive.